What are your requirements? Completely reverse the order, or switch the two pairs of rgb() settings? I assumed switch the pairs...

Didn't see any advantage to using a loop to reconstruct:

local lcInside, laBusted, lcReturn
dimension laBusted[1]

oldval="RGB(0,0,0,255,255,255)"
lcInside = substr(oldVal, at("(",oldVal)+1)
lcInside = left(alltrim(lcInside), len(alltrim(lcInside))-1)

if alines(laBusted, lcInside, ",") = 6
	lcReturn = "rgb(" + laBusted[4] + "," + laBusted[5] + "," + laBusted[6] + "," + ;
	                  laBusted[1] + "," + laBusted[2] + "," + laBusted[3] + ;
	                  ")"
else
	error "invalid source string"
endif
? lcReturn>

>